1. Field of the Invention
The present invention relates to a semiconductor device, a surface emitting diode and an array of surface emitting diodes, which use a light reflecting layer consisting of unit semiconductors whose thickness continuously varies in a direction of thickness of the reflecting layer.
2. Discussion of the Prior Art
A light emitting diode is widely used for optical data communication equipment, displays and sensors. Such light emitting diode includes an active layer for producing light, which is formed on a semiconductor substrate by epitaxy, such as liquid phase epitaxy or vapor phase epitaxy. As one type of light emitting diode, there is known a surface emitting diode adapted to emit light produced by an active layer, from a light emitting surface which is formed substantially parallel to the active layer.
The optical output of a light emitting diode is determined by the internal quantum efficiency upon conversion of an electric energy into an optical energy, and the external quantum efficiency upon emission of light outside the diode. In the surface emitting diode, the reflecting layer is disposed on one side of the active layer which is remote from the light emitting surface, so that the reflecting layer reflects a portion of the light produced by the active layer, by optical wave interference known as Bragg reflection, back toward the active layer, in order to increase the external quantum efficiency for thereby improving the optical output of the diode. The reflecting layer is generally a laminar structure consisting of a plurality of unit semiconductors which are superposed on each other and each of which consists of two or more semiconductor films having different compositions. The specific wavelength of light to be reflected by the reflecting layer is determined by the particular refractive indices of the semiconductor films. Such a reflecting layer is also used in a semiconductor laser. An example of a unit semiconductor of the reflecting layer consists of a AlAs semiconductor film and a Al.sub.x Ga.sub.1-x As semiconductor film. Thicknesses T.sub.A and T.sub.G of these AlAs and Al.sub.x Ga.sub.1-x As semiconductor films are calculated according to the following equations (1) and (2), respectively: EQU T.sub.A =.lambda.B/4n.sub.A ( 1) EQU T.sub.G =.lambda.B/4n.sub.G ( 2)
where,
.lambda.B: center wavelength of light to be reflected PA1 n.sub.A : refractive index of the AlAs semiconductor film PA1 n.sub.G : refractive index of the Al.sub.x Ga.sub.1-x As semiconductor film
The thickness of the unit semiconductor is therefore equal to T.sub.A +T.sub.G.
There is also known a surface emitting diode array which has a common substrate, a plurality of active layers formed on the common substrate, and a plurality of light emitting surfaces each formed on one side of the corresponding active layer which is remote from the substrate. This surface emitting diode array is used as a light source for an image-wise exposing head for a printer, and a display device. Generally, the surface emitting diode array is fabricated by forming p-n junctions on the common substrate, by vapor or liquid phase epitaxy or other epitaxial crystal growth techniques.
The wavelength of the light that can be reflected by the reflecting layer by optical wave interference is limited to a specific wavelength range which satisfies the condition of the optical wave interference. That is, the wavelength range of the light reflected by the reflecting layer is relatively narrow, and depends upon the thickness (calculated according to the above equations) and refractive index of each unit semiconductor. Accordingly, even small amounts of variation in the thickness and change in the composition of the unit semiconductors will cause deviation of the wavelength range of the light reflected by the reflecting layer, from the nominal wavelength of the light produced by the active layer, whereby the optical output of the diode is lowered. This means considerable difficulty in fabricating the diode so as to strictly meet the requirements. In the case of an infrared emitting diode using a GaAs active layer, for example, the ultraviolet radiation produced by the active layer has a wavelength range of about .+-.35 nm whose center is 880 nm. To completely cover this wavelength range, the thickness of the reflecting layer should be extremely accurately controlled. Further, it is difficult to precisely control the epitaxial growth on a large substrate, so as to assure constant thickness of the reflecting layer over its entire surface area. Thus, the yield ratio of the known diode is considerably lowered due to local thickness variation of the reflecting layer.
It is also noted that where each unit semiconductor of the reflecting layer consists of two superposed semiconductor films having different compositions, different lattice constants of the two semiconductor films may cause crystal defects due to lattice mismatch, resulting in an increase in the number of crystal defects such as dislocation of the active layer formed by epitaxy on the reflecting layer. The dislocation of the active layer may grow under heat during operation of the relevant semiconductor device (e.g., surface emitting diode), leading to so-called "dark line deterioration" of the active layer, thereby shortening the expected service life of the device. The use of the two semiconductor films has another drawback that the discontinuity of bands at the interface of the two semiconductor films increases an electrical resistance value, requiring the device to be operated with a higher voltage, as compared with a device which does not have a reflecting layer.
The conventional surface emitting diode has only one active layer, and is not capable of emitting lights having different center wavelengths. For permitting the diode to emit radiations having respective different center wavelengths, it is considered to provide the diode with two or more active layers that are superposed on each other. In this arrangement, however, a radiation produced by one of the active layers is more or less absorbed by another active layer, and a radiation is produced by this active layer upon absorption of the radiation produced by the above one active layer. Consequently, the output level of the emitted light having the desired wavelength is not sufficiently high.
The conventional surface emitting diode array may suffer from difference in the intensity of the radiations emitted by the individual diodes. This difference may result in undesired variation in the density of images printed by a print head if the diode array is used as a light source of the print head for image-wise exposing a recording medium or a photosensitive medium (such as a photoconductive drum). To solve this problem, it is considered to use suitable resistors in the driver circuit for the diode array, so that the radiations emitted by the individual diodes have the same intensity. However, this solution inevitably makes the print head complicated. Moreover, the solution requires an increased power consumption to assure output uniformity of all the diodes, since the resistors are used to lower the original output values of the diodes except one diode whose original output value is the smallest, to the smallest output value.
The conventional surface emitting diode array also suffers from reduction in the intensity of the emitted radiations, because portions of the radiations produced by the diodes are absorbed by the substrate or other elements. This problem is serious particularly where the surface area of the light emitting surface for each diode is extremely small, as in the case of a surface emitting diode array used as a light source for a print head as indicated above, wherein multiple diodes are arranged on the substrate. If the intensity of the emitted radiations is not sufficiently high, the amount of image-wise exposure of a recording or photosensitive medium is insufficient, whereby the resolution of printed images such as characters is accordingly lowered.